What is your diagnosis? Pleural fluid from a neonatal Thoroughbred filly with pneumonia.

A 3-day-old filly was presented to the Cornell University Hospital for Animals with an umbilical hematoma and mild aspiration pneumonia. The foal underwent abdominal surgery for resection of the hematoma. Recovery was uneventful, but 3 days after surgery, the foal became progressively tachypneic. Imaging studies revealed bilateral pleural effusion and pleuropneumonia. Cytologic evaluation and bacterial culture of the pleural fluid from both sides of the chest revealed sterile exudates, consisting mostly of neutrophils, with fewer macrophages and lymphocytes. Pleural fluid macrophages contained variable amounts of purple-magenta globular material in their cytoplasm. A lighter colored granular precipitate was also seen throughout the background of the smears. Similar material was identified in a macrophage in a peripheral blood smear prepared 2 days after abdominal surgery. Large amounts of extracellular pink precipitate were also seen in the blood smear and persisted in the blood for 7 days after surgery. A protective lubricant, carboxymethylcellulose, had been instilled into the abdominal cavity during surgery to prevent intra-abdominal adhesions. The intracytoplasmic pigment within pleural fluid and blood macrophages and the extracellular precipitate in peripheral blood and pleural fluid smears was compatible with carboxymethylcellulose. The material was probably derived hematogenously and was considered an incidental finding. The pleuritis was attributed to exacerbation of the original aspiration pneumonia by the general anesthesia.